Hearing and system



June 8, 1943. w. DUBILIER HEARING AID SYSTEM Filed April 29, 19-41 2 Sheets-Sheet 1 FTC-3.2.

R O T N E V m ATTO R N EY Jung 8, 1943. w. DUBILIER 2,321,370

HEARING AID SYSTEM Filed April 29, 1941 2 Sheets-Sheet 2 :kmlllllhlli- Jr" H'Br' +j i V 'INVENTOR fie, v

ATTORNEY Patented June 8, 1943 William Dubilier, New Rochelle, N. Y., assignor,

by mesne assignments, to Patents Research Corporation, New York, N. Y., a corporation of New York Application April 29,1941, Serial No. 390,858

3 Claims.

' Thepresent invention relates to improvements in hearing aids which are miniaturetelephone systems comprising a microphone transmitter for picking up and transforming sound energy into electrical energy, an amplifier to raise the ampli-' tude of the energy supplied by the microphone and one or a pair of earphone receivers worn by 'the user andenergized by the output of the amplifier.

In apparatus of this type, the size and weight play a great and vital part and should be kept as small as possible to enable the device, inclusive of operating batteries, to beworn without any too great inconvenience to the user.

-With the type of microphones and electron tube amplifiers at present being used in hearing aid apparatus, various difiiculties are encountered which either impair the quality and operating stability of the devices or which will require a high amplification of the initial energies delivered by the microphone resulting in undesirable increase of weight and bulk.

There are two types of microphones in use generally in hearing aid apparatus at present, both of which possess drawbacks and disadvantages in one or the other respect. Carbon microphones used in hearing aids deliver a comparatively large current and accordingly require little amplifica-' tion, one amplifying'tube being .suflicient in many cases where the hearing loss is small, thus fulfilling the requirement of low weight and bulk. Such microphones, however, have the disadvantage of being extremely noisy which not only will impair the intelligibility of the sound output but may become so annoying as to be more a burden and inconvenience rather 'than'an aid and enjoyment to the users.

For the above reason, piezoelectric microphones have come into use which, while substantially free of the objection of internal noise, have the disadvantage that the energy generated by the piezoelectric effect is so small that substantial amplification, in practice by means of at least three amplifying stages in cascade, will be required resulting in high weight and'increased crease and eventually the coupling may become .of the function and operation of the high enough to sustain feedback oscillations. These oscillations which are in the audio frequency range can be so disturbing that replacement of the batteries may be necessary before their otherwise useful life is exhausted, or separate isolated batteries will have to be used which are objectionable from the standpoint of weight and bulk.

The present invention relates to a novel hearingaid system utilizing an electrostatic microphone as a sound pickup device in connection with a special electrical translating or amplifying circuit which not only makes available to its fullest extent theusel'ul characteristics of the condenser microphone such as small size and weight as well as low noise level, but which will enable the use of a limited number, including a single translating tube responsive to electrical phase rather than amplitude changes effected by the vi bratory movement of the microphone diaphragm to produce a suflicient output current for energizing the earphone receiver. Experiments and investigations made have shown that the-microphone may have the size of an ordinary coat button and still produce sufficient capacitance changes to ensure adequate output current variations of a single tube. Since the input and output circuits of the translating tube are substantially decoupled exteriorly and since the control.

Other objects and advantages of the invention 1 will become more apparent from the following detailed description taken with reference to the 'accompanylng drawings forming part of thi specification and wherein:

Figure 1 is a circuit diagram for a complete hearing aid system embodying the principles of the system,

Figure 2 is a theoretical diagram explanatory system shown in Figure 1,

Figure 3 is a circuit diagram for a simplified hearing aid system according to the invention,

Figures 4 and 5 are front and side views, respectively, the former with the cover removed, of a preferred structural embodiment of a hearing aid device according to the invention, 'and Figure 6 is a circuit diagram for the hearing aid shown in Figures 4 and 5.

Like reference characteristics identify like parts throughout the diiferent views of the drawings. I l

The function and novel aspects characteristic of the invention will be further understood from the following: In both the carbon and piezoelectric microphones in use at present, electrical energy is either generated or amplitude modulated in accordance with the vibrations of the microphone diaphragm; that is, in turn the pressure variations of the sound waves. In order to apply this energy to the grid of a standard amplifying tube, a varying current flow has to be set up through a load impedance connected in series with the microphone to develop a voltage drop to be impressed upon the input grid of the amplifier. As a result, in these arrangements either heavy currents are required initially as in the case of the carbon microphone attended by noise and other defects or high degrees of amplification must be employed if only small initial energies are available as in the case of the piezoelectric microphone.

The system according to the present invention, on the other hand, is based on phase changes rather than amplitude changes caused by the minute variations of the capacitance of a condenser microphone of small-dimensions, which phase changes by the proper choice, design and correlation of the constants of the translating circuit are caused to cover a range suillclent to utilize substantially the full swing of the plate output current of a single tube, limited only by the current carrying capacity of the tube and the permissible distortion.

Referring more particularly to Figure l, I have shown therein a circuit for a hearing aid system embodying the principles of the invention. An electron discharge tube II, in the example shown a tube of the pentode or similar type, is provided with the following electrodes arranged in the order named: a cathode or source of electrons ll of any suitable type, a first control grid 12, a second control grid IS, the latter being enclosed by a screen grid H, a suppressor grid l5 connected internally to the cathode, and an anode or plate [6. The suppressor grid may be omitted without materially affecting the operation of the system. The screen grid II and plate I! are connected to suitable sources of high potential, in the present case to taps of a dry cell battery as indicated by the plus signs. As a result, a steady electron discharge stream emitted from the cathode will fiow to the screen and plate electrodes. This electron stream is initially modulated in accordance with a high frequency operating potential of substantially invariant amplitude and constant frequency supplied by any suitable source or oscillator and impressed uponthe grid 12 and cathode II. The oscillator shown in Figure 1 is of the crystal control type of known design comprising a three element tube l1 having a resistance shunted piezo-crystal I8 connected to the grid and cathode thereof and being provided with a parallel tuned circuit resonant to the crystal frequency and inserted in the plate circuit of the tube. The oscillations are impressed upon the grid l2 through coupling coil 2| in inductive coupling relation to the induction coil of the tuned circuit 20.

The second control grid I3 is operated at cathode potential or at a potential negative with respect to the cathode or any other potential reference points such as the chassis of the apparatus. This grid is connected to the cathode through a resonant impedance consisting in the example shown of a parallel tuned circuit comprising an inductance 22 and a condenser 23. The grids I2 and IS, in the example shown, are biased negatively with respect to the cathode by the provision of a condenser shunted resistance 24 in the cathode-to-ground lead.

A condenser microphone 26 of small siz is connected in parallel to the tuned circuit 22, 23 so as to form a part of the effective tuning capacity of the circuit. The microphone construction shown comprises a cup-shaped base 36 having mounted therein an annular member 3| providing a circular knife edge support for the diaphragm 32 of very thin metal foil covered on both sides with thin insulating layers such as sheets of silk and having its outer edge clamped between the member 30 and an insulating washer 33. The diaphragm is tensioned by means of a further mounting ring 34 having a knife edge extension by means of a further mountin ring 34 having a knife edge extension engaging the inner portion of the diaphragm and acting as a spacer for the perforate cover electrode 35 forming a cooperating condenser electrode together with the diaphragm 32., All the elements are clamped into firm engagement in any suitable manner to properly tension the diaphragm v and to provide a sealed air pocket or cushion 38 between the base 30 and the diaphragm 32. The cushion-36 provides a resilient restraining or control force for the movement of the diaphragm 32 which has a practically negligible mass. A pair of lead wires extending from the diaphragm 32 and cover electrode 35 are firmly clamped between the elements and passed through a flexible insulating sheath or cord to the resonant circuit 22, 23. The microphone is shown provided with a clip 38 for attaching the same to a buttonhole or coat pocket and may be inserted in a protective insulating casing such as shown in Figure 5 or in any other suitabl manner.

In a system of the type shown in Figure 1 and described in greater detail in U. S. Patent No. 2,208,091, a so-cal1ed virtual cathode or concentrated electron space charge will be formed adjacent to the control grid l3 by electrons accelerated by the screen grid from the cathode and subsequently repelled or decelerated by the negative field extending from grid l3 so as to become massed in front of this grid and to form an electron cloud of increased volume density known as a concentrated space charge or virtual cathode. The volume density of this virtual cathode varies in the same manner as the spac current; that is, in accordance and in phase with the potential on the control grid l2 supplied by the oscillator.

Due to the presence of this variable space charge or virtual cathode, a displacement current will .be induced in the external circuit connected to the grid l3 which displacement current will vary in accordance with the rate of change of the space charge fluctuations. Accordingly, the phase of the induced current will be lagging by the phase of the space charge variations; that is, in turn the potential impressed on the grid l2. This displacement current causes a potential drop to be developed through the external circuit 22, 23, 26, resulting in a control potential upon the grid l3 affecting the electron space current conveyed to the plate IS.

The phase of the potential developed upon the grid l3 will depend on the character of the impedance of the external circuit as follows: if the resonant frequency of the external circuit 22, 23, 26 equals the frequency of the impressed potential supplied by the oscillator, its impedance will be purely resistive and accordingly the potential drop will be in phase with the displacement current, resulting in a control potential on grid 13 lagging by 90 the potential on grid I2. If the resonant frequency of the circuit 22, 23,, 2B is higher than the frequency of the oscillator due to a decrease of the capacitance of the microphone 26, the circuit will offer inductance impedance thereby causing the potential on grid I3 to be les than 90 with respect to the potential on grid I2 and resulting in an increase of the steady or average plate current component. Vice versa, if the resonant frequency of the circuit 22, 23, 26 is less than the frequency of the oscillator due to an increase of the microphone capacitance the circuit will offer capacitative impedance to the displacement current and accordingly the potential on grid I3 will have a phase greater than 90 relative to the potential on grid 12, resulting in a decrease of the average plate current component.

From the foregoing it is seen that a dual control of the electron space current passing from the cathode to the plate 16 will be effected by the grids l2 and lit in the rhythm of the same (operating) frequency but with a varying relative phase between the control potentials depending upon the departure of the resonant frequency of the circuit 22, 23, 26 from the oscillator frequency. As a resultof this dual control, the average plate current is subjected to variations proportional to the cosine of the relative phase angle between the control potentials on the grids l2 and I3 as can be shown by a detailed analysis. relative phase angle of the control potential is 90; that is, if the circuit 22, 23, 26 is in tune with the oscillator frequency, the variable component of the steady plate current will be zero; that is, the average plate current will be equal to the steady or quiescent discharge current of the tube. Whenever the frequency of the circuit 22, 23, 28 departs in either sense from the oscillator frequency, the average plate current will increase or decrease substantially linearly with this frequency departure over a predetermined operating range asshown at X-Y, curve A in Figure 2 representing a plot of the average plate current ip vs. the microphone capacitance C. In Figure 2,10 represents the steady or quiescent plate current correspondingrto the center or average capacity Co of the condenser miscrophone in its equilibrium position. When sound waves strike the microphone diaphragm its capacity will increase or decrease within a range 1, resulting in a corresponding change of the average plate current between points X and Y of curve A. This change of the average plate current is further dependent on the Q value of the circuit 22, 23, 26 and it has been found that the higher the Q the steeper will be the slope of the curve and the greater the current variations obtained in the plate circuit. A high Q however will result in a decrease of the linear operating range of the curve; that is, the maximum capacity swing which may be translated into plate current variation without distortion. This is illustrated in Figure 2 by comparison with curve A showing the plate current variations for a relatively lower Q" compared with curve A (maximum capacity swings r and 1'', respectively) According to the present invention, the Q" of the resonant circuit or equivalent resonant Thus, if the impedance means connected to the grid I3 is made sufficiently high by using an operating frequency and an inductance for the circuit so correlated with the operating constants of the tube and remaining circuit elements as will be compatible with the mechanical and other operating require ments for the condenser microphone, to afford a substantial, preferably a full, swing of the average plate current over the linear operating range of the characteristic in accordance with the capacity changes of the microphone, limited only by the permissible distortion and to current carrying capacity of the tube. Thus considering an amplifier tube of given output capacity operated in the ordinary manner by electrostatic control action, the minute energies supplied by the microphone will enable only a fraction of the possible plate current swing, thus making necessary the use of a second and third amplifier stage in order to utilize the full linear plate current range and o deliver a sufiicient final output current for.

' energizing the earphone.

In the arrangement according to the invention, the same effect can be obtained with a single tube by the proper construction and correlation of the operating constants to utilize substantially the full plate current swing in the manner described since the control is due to the phase relation between two control potentials rather than the amplitude of a single potential as in the standard amplifier. In Figure 1 the plate current variations are utilized to operate an earphone receiver 21 by way of an audio frequency transformer 28. If desired, a further audio amplifier stage may be provided between the converter and the earphone 21.

In the system shown in Figure 1, the average screen grid current undergoes a variation similar to the plate current and accordingly it is possible to utilize the screen current variations for energizing the earphone receiver in substantially the same manner by a suitable coupling arrange ment in the screen grid circuit. It is furthermore possible to interchange the oscillator and the resonant circuit 22, 23, 26 without materially affecting the operation and function of the system. The condenser 23 may be omitted, in which case the microphone 26 will act as the sole tuning capacity in addition or without the capacity between the connecting leads to the microphone and the internal capacity between grid l3 and the cathode. As is understood, the oscillator may be of any suitable type known.

Practical tests with amplifying and translating circuits of the above type have shown that a microphone having an effective electrode area of about 1 cm.; that is, in actual size about onehalf of that shown in Figure 1 and a spacing be tween the electrodes of about 1 mm. will provide sufficient capacitance variations (from about 5 to 10 hundredths of a micromicrofarad) in a hearing aid using an operating frequency of from one to about 5 megacycles and a tuned circuit with a fairly high Q," to enable the use of a single tube for cases of slight hearing loss and to provide sufficient output by using an additional amplifier stage as shown in Figure 6 sufficient for average cases in practice. If a microphone construction is used such as shown in Figure 1, the spacing between the diaphragm 3-2 and cooperating electrode 35 may be as small as mm. resulting in a substantially increased capacitance variation and sufllcient output current obtained in a single tube for energizing the earphone receiver which may be both of the air or bone con- -duction type. Such a small microphone may be made in the form of a coat button such as to be actually inconspicuous and may be worn in the place of an ordinary button or otherwise inconspicuously combined with a badge, pin or the like worn by the user.

Referring to Figure 3, I have shown a simplified circuit for a hearing aid system according to the invention wherein the oscillator and discriminating tubes are combined, resulting in a further reduction of weight and bulk. Tube l shown is similar to the tube of Figure 1 except for the omission of the suppressor grid which is not essential for the operation of the circuit. Each of the control grids l2 and I3 are operated at cathode potential or biased negatively with respect to the cathode so that no steady or direct electron current will be conveyed to these electrodes. These grids are further connected to the cathode through resonant circuit normally tuned tosubstantially the same frequency. The screen grid vl4 is maintained at a high positive potential with respect to the cathode and bypassed to the latter through a condenser to prevent any alternating or oscillating potential from being developed thereon. The resonant circuit connected to grid I2 and the cathode is of the series-parallel tuned type comprising in the example shown a condenser 40 shunted by a series tuned path comprising an inductance 4| in series with a condenser 42. The latter is shunted by the condenser microphone 43 which may be of the same type as shown in Figure l. A resistor 44 connected across the grid l2 and cathode serves to provide a direct current return path for this grid. A pair of wires 45 connect the inductance 4|, condenser 42 and microphone 43 which are combined into a unit with the discriminator tube. The resonant circuit connected to the grid I3 is a normal parallel tuned circuit comprising condenser 23 shunting an inductance 22 similar to the circuit provided in Figure 1.

Disregarding temporarily in Figure 3 the presence of the anode IS, the system associated with the grids l2, l3 and I4 constitutes an oscillator based on the presence of a forward coupling from grid l2 towards grid l3 and a return or feedback coupling from the latter towards the former by way of the variable space charge or virtual cathode formed near the grids, there being no other external coupling between the grid circuits which are carefully decoupled by means of metallic screens.

The function of this oscillator will become further apparent from the following: an alternating potential impressed upon grid l2 will cause a potentialof the same frequency to be developed on grid l3 through variable space charge coupling by inducing a displacement current through the external circuit 22, 23. If the latter is of purely resistive character as in the case of a tuned circuit, the potential developed on grid I will be lagging the potential on grid l2 by exactly 90. Similarly, if a potential is applied to grid IS a potential of like frequency will be developed on grid |2 through space charge coupling also displaced by 90 with respect to the former if the external circuit connected to the grid I2 is resoii'ant to the frequency of the impressed potential.

In the latter case, however, the developed potential on grid l2 will be leading the impressed potential by 90 Thus an initial potential due to a dampedoscillation in the circuit of grid l2 caused by closing a switch will result in a lagging quadrature potential on grid l3 which in turn will cause a leading quadrature feedback potential to be developed upon grid l2. Accordingly therefore, this feedback potential will be in phase with the initial oscillation thereby sustaining the latter, provided the Q" of the circuit is of a suitable value so that the direct current power supplied through the grid I3 is sufllcient to overcome the energy losses.

In a system as shown in Figure 3, it has been found that by detuning one of the circuits such as the circuit connected to grid |2 by the capacitance variation of the microphone 43 in which case the phase displacement of the feedback potential will become greater or less than 90, the oscillations will continue as long as there is an in-phase feedback component suflicient to overcome the losses in the circuits. As a result, the electron space current passing to the anode I6 is subjected to a dual control by the same control potential but with varying relative phase in a manner similar as in the case of Figure l, whereby the average plate current will undergo a similar variation as shown by curves A and A in Figure 2.

The reason of using a series parallel tuned circuit in Figure 3 is to overcome the effect of the self capacity between the connecting wires 45 which if excessive would swamp the minute variations of the microphone capacity. By'the employment of an arrangement as shown a comparatively large capacity including the capacity of condenser 40 plus the line capacity is shunted by the differential of two reactances of opposite sign: that is, the inductance 4| and the capacity of condenser microphone 43 shunted by further condenser 42. In this manner, small variationsof the microphone capacity will result in large changes of tuning or phase shift substantially independently of the line capacity. If the microphone is mounted close to the tube as in Figures 4 to 6, this precaution is not necessary.

Referring to Figures 4 and 5, I have shown a preferred constructional form of a hearing aid apparatus according to the invention. The tubes and other circuit elements are mounted in an oblong tubular housing preferably of phenol or other plastic material having a lower half 50 and upper removable half 5 A pair of miniature vacuum tubes 52 and 53 one of which serves as the discriminator and the other as an additional amplifier (see circuit diagram, Figure 6) are mounted at opposite ends of the housing with the metal shielded resonant circuits and audio frequency transformer indicated by dotted rectangles 56, 51, 58, respectively, occupying the space between the tubes 52 and 53. The remaining elements such as by-pass condensers and resistors and connecting wires may be suitably arranged in the spaces between the parts. The microphone 55 is removably attached to the front 01' the housing by a plug and socket connection as indicated and has mounted thereon a variable volume control resistor comprising a resistance element 60 in the form of a spirally wound wire or the like and a cooperating sliding contact carried by a knurled operating member rotatably mounted upon the microphone casing. The circuit diagram shown in Figure 6 includes the discriminator tube III which is of substantially the same type as shown in Figure 3 with the exception that the microphone 43 is directly connected across the tuned circuit 40, 4| due to its mounting directly adjoining the discriminator tube. The output currents are amplified by a further audio frequency amplifier tube 68 coupled to the discriminator tube byway of a resistance coupled network 85 of known construction and applied to the earphone 21 by way of output transformer 28 and volume control resistor 46. The discrim inator tube in the example shown being of the directly heated filamentary type as is customary in small miniature tubes, is further shown provided with a so-called space charge grid 61 located close to the cathode and biased positively by connection ton suitable tap of the anode battery 6!. In this manner the eii'ect of the varying voltage drop through the filament on the space charge is substantially equalized by the formation of a further virtual cathode between the grid 61 and control grid I2 which forms the primary electron source for the discharge stream.

Qther details of the circuit are similar to those sh'own in the previous illustrations and will be understood by those skilled in the art.

7 It will be evident from the foregoing that the invention is not limited to the specific construction and arrangement of parts shown and dis closed herein for illustration. but that the underlying novel principle of the invention will be susceptible of numerous variations and modifications coming within the broader scope and spirit thereof as defined in the appended claims. The specification and drawings are accordingly to be regarded in an illustrative rather than in a limiting sense.

I claim:

1. A compact small hearing aid apparatus adapted to be worn inconspicuously; said apparatus comprising a condenser microphone having a pair of spaced electrodes movable relatively to each other in accordance with sound impulses impinged thereon -to change the capacitance thereof; the electrodes of said microphone each having an effective surface area of the order of one square centimeter; a receiver member; a translation system connecting said microphone with said receiver; said translation system comprising an electron discharge tube having at least a cathode, an output electrode, a control grid, and means for initially varying the electron space current to said output electrode according to a predetermined invariant frequency of substantially constant amplitude, the output current of said tube being variable over a predetermined range, means for producing a concentrated electron space charge adjacent to said control grid, resonant impedance means including said microphone tuned to said predetermined frequency in the quiescent position of said microphone, said resonant impedance means being connected to said control grid and cathode to develop a control potential upon said grid by capacitative coupling with said space charge having a varying phase relation to said'predetermined frequency to produce current variationsin said output circuit substantially proportional to the capacitance changes of said microphone in accordance with said sound impulses, said predetermined frequency and the constants of said resonant impedance means being so correlated I as to efiect current variations by said microphone over substantially the full output current range of said tube. A

2. A. compact small hearing aid apparatus adapted to be worn inconspicuously; said apparatus comprising a condenser microphone having a pair of spaced electrodes movable relatively to each other in accordance with sound impulses impinged thereon to change the capacitance thereof; the electrodes of said microphone each having an effective surface area of the order of one square centimeter; a receiver member; a translation system connecting said microphone with said receiver; said translation system comprising an electron discharge tube having a cathode, a positively biased space charge grid near said cathode a first control grid, an accelerating grid, a-second control grid and an anode all arranged in the order named, the output current of said tube being variable over a, predetermined range, a pair of resonant circuits one of which includes said microphone being connected each to one of said control grids and said cathode and being tuned to substantially the same frequency in the quiescent position of said microphone to the maintained sustained oscillations super-imposed upon a steady stream through said tube and to produce current variations in the anode circuit substantially proportional to the capacitance changes of said microphone, the frequency of said oscillations and the constants of said resonant circuits being so correlated as to effect anode current variations of said microphone over substantially the full output current range of said tube.

3. A compact small hearing aid apparatus adapted to be worn inconspicuously; said appa-v ratus comprising a condenser microphone having a pair of spaced electrodes movable relatively to each other in accordance with sound impulses impinged thereon to change the capacitance thereof; the electrodes of said micro hone each having an effective surface area of the order of one square centimeter; a receiver member; a.translation system connecting said microphone with said receiver; said translation system comprising an electron discharge tube having a directly heated cathode, a space charge grid ad- I jacent to said cathode, an output electrode, a control grid and means for initially varying the electron space current to said output electrode at a predetermined high operating frequency, means for producing a concentrated electron space charge adjacent to said control grid, resonant impedance means including said microphone as an effective tuning element connected to said control grid and cathode to develop control potential at operating frequency upon said grid by capacitative coupling with said space charge, an output current connected to said output electrode, and means associated with said output circuit for utilizing energy varying in amplitude in proportion to said frequency relation, said last-mentioned means being connected to said receiver.

WILLIAM DUBILIER. 

